In circuit-based transport networks, such as SDH/SONET networks, automatically restoring networks (such as ASON-operated networks, or DWDM networks) is well established, and sophisticated transmission protection schemes allow for service restoration in timescales often better then 50 ms. These timescales are not critical, even if packet services, such as encapsulated ATM or Ethernet traffic, are transported across those underlay networks, even if the overlay packet networks utilize network protection and restoration schemes like the Spanning Tree Protocol, because the restoration times of the transport network are well below protocol timeouts of the packet networks, and hence avoid lengthy reconfiguration timeouts caused by recovery of the active topology in a Spanning Tree network.
When operators choose to install efficient access and aggregation networks for residential services delivery, plain Ethernet is very often the transport technology of choice. Missing the carrier grade network protection capabilities of the circuit based technologies, the experienced quality of the delivered service becomes easily endangered, e.g. in case of multimedia services requesting uninterrupted data transmission. Similar situations can show up for other ‘mission critical’ applications with high resiliency demands, such as grid computing, storage area networks, remote backup services and the like.
While network elements themselves offer some degree of equipment protection in the described situations, network protection that avoids lengthy recovery, like STP recovery, MAC address relearning and the like are desirable. In packet-based networks, Link Aggregation can be used for this purpose.
In current Ethernet access networks, such as residential access networks, internet access services have been a primary focus of network providers, with some attention to growing VoIP (Voice over IP) services. Meanwhile, due to the advent of broadband video distribution to residential end-customers over packet-based infrastructure (IPTV, e.g. broadcast TV, Video on Demand (VoD) etc.), operators need to accommodate increasing bandwidth demands in metro access networks. Video distribution also imposes stringent Quality of Service (QoS) demands and availability requirements onto the service infrastructure such that the connection between the transport network and the access network element (e.g. a digital subscriber line access multiplexer or DSLAM) forms a bottleneck in terms of the offered bandwidth to satisfy the total of end-subscriber demands of TV channels.
One way of increasing bandwidth towards the edge network element is aggregation multiple parallel links of the same technology. With the advent of next generation DSLAMs, Gigabit Ethernet is deployed and some access nodes can aggregate multiple links to form a single logical port, while higher speed technology like 10 Gigabit Ethernet is typically not available.
The Institute of Electrical and Electronic Engineers (IEEE) has promulgated the IEEE 802.3-2005 (and predecessor) standard which provide for a link aggregation control protocol (LACP). According to LACP, a single logical link can be formed using one or more physical links. When, in a bundle of n physical links, a subset of these links fails, the LACP distribution mechanism will distribute the traffic load across the remaining operational links, independent of the services provided. However, the nature of this distribution function is not standardized within IEEE.
According to the LACP, it is possible to use a special option that can declare one or more links as standby links. However, the protocol also requires that the declared standby links are inactive (i.e., no traffic may flow) until the standby link(s) are declared to be active, such as when an active link has failed and it is necessary to move the traffic over to a previously declared inactive link. Unfortunately, this requires that the nodes on both sides of the link use the same algorithms and therefore make the same decision regarding which backup link to use, which takes time to accomplish. Moreover, the standard inhibits efficient utilization because standby links do not transmit active traffic.